Presently, x-ray detectors in computerized tomography find use in both medical and industrial applications. In such applications, it is highly desirable for the detector to have a high operating efficiency. The efficiency of the detector is determined primarily by the percentage of incident x-ray energy that is detected. Detector efficiency may be improved by increasing the density of the detecting medium. As density of the detecting medium is increased, incident x-ray energy is absorbed in a shorter distance of propagation through the medium. In present medical and industrial detectors, x-ray energy is detected on the basis of its interaction with a gaseous detecting medium. The interaction produces electrons and positive ions which propagate between the proximate voltage and collector plates under the influence of an electric field applied therebetween. The electrons or positive ions, depending on the direction of the electric field, detected at the collector plates provide the information from which a computerized tomography image is constructed. Thus, detector efficiency is maximized when all of the electrons and positive ions liberated by the incident x-ray energy are detected. However, a portion of incident x-ray energy not immediately interacting in the gaseous detecting medium may be lost by being absorbed in traversing the plate structure within the detector. For example, in a preferred embodiment of the x-ray detector for medical computerized tomography described in commonly assigned U.S. Pat. No. 4,031,396 to Whetten et al., a plurality of voltage and collector plates comprising high molecular weight material such as tungsten are disposed radially with respect to the incident x-ray beam. X-ray energy incident on these tungsten plates is absorbed without interacting with the detecting medium to liberate electrons and ions, and thus detector efficiency is less than maximum. If density of the detecting medium in such a detector is increased, the absorption length of the incident x-rays will be shortened and a greater percentage of the incident x-rays will interact with the medium before striking and being absorbed by the plates. Further, the shorter absorption length of the incident x-rays permits construction of a detector with shorter collector plates. As a result, the plates present a smaller plate surface area capable of absorbing incident x-rays.
Increasing the detecting medium density also has the beneficial effect of decreasing the absorption length of fluorescent x-rays. The atoms of a gaseous detecting medium, upon interacting with the incident x-ray energy, may themselves emit low energy x-ray photons. When the detecting medium allows a long absorption length for a fluorescent x-ray photon, the photon thus generated by secondary emission may be detected within the detector at a location remote from the point where the incident x-ray entered the detector. Such event degrades spatial resolution and decreases detector efficiency since a portion of the incident x-ray energy is not usefully detected.
One solution known in the art to the fluorescent x-ray secondary emission problem is to use collector and voltage plates comprising a high atomic weight material, such as tungsten, as described in the above-cited patent. The plates absorb the fluorescent x-rays before they travel any significant distance within the detector. This technique results in absorption of the fluorescent x-rays by the plates, thereby avoiding the aforementioned degradation of spatial resolution caused by the fluorescent x-rays; however, detection of that portion of the incident x-ray energy responsible for the fluorescent x-rays is precluded and thus detector efficiency is decreased. Since fluorescent x-rays have an absorption length that is inversely proportional to density of the detecting medium, a solution to this dilemma is to increase the medium density. Moreover, using a detecting medium density selected to provide a fluorescent x-ray absorption length on the same order of magnitude as the spacing of the individual collector plates within the detector avoids need for the voltage and collector plates to be of an absorptive character. Thus, the fluorescent x-rays can be detected in close spatial proximity to the point where the incident x-ray enters the detector. As a result, efficiency of the detector is enhanced.
Both the medical and industrial x-ray detectors known in the art utilize high atomic weight noble gases, such as xenon or argon, as a detecting medium. In such case, a density increase is accomplished by increasing the gas pressure. The increased gas pressure, however, requires a more massive detector housing so as to be structurally capable of reliably withstanding the higher gas pressure. The medical x-ray detector disclosed in the above-noted U.S. Pat. No. 4,031,396 employs gaseous xenon at a pressure in the range of 10 to 50 atmospheres. A typical operating pressure of 25 atmospheres corresponds to a gaseous xenon density of approximately 0.16 grams per cubic centimeter. An increase in gas pressure of the detecting medium beyond 75 to 80 atmospheres would necessitate a redesign of the detector housing in order to enable containment of the higher pressure gas. Industrial detectors, such as the one described in U.S. Pat. No. 4,394,578, may employ detecting medium gas pressures of up to 200 atmospheres. At such pressures, one might assume that the detecting medium density is high enough to avoid degradation of detector efficiency by detecting fluorescent x-rays at a point remote from where the incident x-ray enters the detector. However, x-rays utilized in industrial tomography are typically far more energetic than those in medical tomography, and thus have significant absorption lengths even in a very high pressure gaseous medium. Further, as seen in the abovenoted patent, the industrial detector requires considerable structural enhancement of the housing to reliably contain the higher pressure gas. Such high pressures, moreover, result in decreased propagation speed of the electrons and positive ions being detected, causing an increase in response time of the detector, defined as the time required for all charged particles (i.e. electrons and positive ions) attributable to a particular incident x-ray beam to be cleared from the detecting medium.